The Cooperative Clinical Study of Dietary Modification on the Course of Progressive Renal Disease is a multi-institutional randomized clinical trial designed to assess the following goals: (1) To compare and investigate the efficacy of a low protein diet and phosphorus restriction with that of a very low protein diet with dietary supplements on the course of progressive chronic renal disease, (2) to determine patient acceptance of these dietary interventions, and (3) to evaluate the nutritional safety and nutritional and other effects of the various dietary interventions. The study has completed the first of four stages, the Protocol development stage and has initiated the second stage, the period of testing the feasibility of the Protocol and Manual of Operations. The Data Coordinating Center is responsible for such items as: (1) distribution of the Protocol, Manual of Operations and Study Forms, (2)in conjunction with the appropriate study committees, revising these documents, (3) training new Data Managers at the Clinical Centers, (4) random assignment of patients to study diets, (5) maintenance of study files,(6) review on a regular basis of all data transmitted by the Clinical Centers, (7) preparation of periodic reports describing the progress of the study, and (8) administrative support of the various study committees. In addition, the following central units will be subcontractors to the Data Coordinating Center: the Nutrition Coordinating Center, the Center for Compliance Enhancement, the Central GFR Laboratory, the Drug Distribution Center, the Central Biochemistry Laboratory, and the Central ECG Laboratory. The Nutrition Coordinating Center will monitor the dietary component of the study and evaluate dietitians' problems in prescribing and counselling activities. The Center for Compliance Enhancement would assist in the development of nutritional intervention and assessment methodologies needed for the effective and efficient conduct of the trial, and to coordinate the implementation of intervention and compliance enhancement activities. The GFR Central Laboratory will be responsible for analyzing specimens for glomerular filtration rate. The Drug Distribution Center will distribute to the Clinical Centers in appropriate packages the keto acids and vitamin supplements. The Central Biochemistry Laboratory will measure specimens sent by the Clinical Centers for various serum chemistries, urine chemistries, and glycosylated hemoglobins. A program of internal quality control will be conducted as well as an external quality control program for the Clinical Centers.